Chrysler RAM TRUCK 2500 1994 Service Manual page 165

6 - 6 SERVICE BRAKE DIAGNOSIS
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Chatter
Brake chatter is usually caused by loose or worn
components, or glazed/burnt lining. Rotors with hard
spots can also contribute to chatter. Additional causes
of chatter are out of tolerance rotors, brake lining not
securely attached to the shoes, loose wheel bearings
and contaminated brake lining.
Brakelining Contamination
Brakelining contamination is usually a product of
leaking calipers or wheel cylinders, driving through
deep water puddles, or lining that has become cov­
ered with grease and grit during repair.
Wheel and Tire Problems
Some conditions attributed to brake components
may actually be caused by a wheel or tire problem.
A damaged wheel can cause shudder, vibration and
pull. A worn or damaged tire can also cause pull.
Severely worn tires with very little tread left can
produce a grab-like condition as the tire loses and re­
covers traction.
Flat-spotted tires can cause vibration and wheel
tramp and generate shudder during brake operation.
A tire with internal damage such as a severe bruise
or ply separation can cause pull and vibration.
MASTER CYLINDER/POWER BOOSTER TEST
(1) Start engine and check booster vacuum hose
connections. Hissing noise indicates a vacuum leak.
Correct any leaks before proceeding.
(2) Stop engine and shift transmission into Neu­
tral.
(3) Pump brake pedal until all vacuum reserve i n
booster is depleted.
(4) Press and hold brake pedal under light foot
pressure.
(a) I f pedal holds firm, proceed to step (5).
(b) I f pedal does not hold firm and falls away,
master cylinder is faulty (internal leakage).
(5) Start engine and note pedal action.
(a) I f pedjal falls away slightly under light foot
pressure then holds firm, proceed to step (6).
(b) I f pedal is effort is high, or no pedal action is
discernible, power booster or vacuum check valve is
faulty. Install known good check valve and repeat
steps (2) through (5).
(c) On diesel models, vacuum pump hose or
pump component may have malfunctioned. Check
pump output with vacuum gauge and repair as
necessary. Refer to service procedures i n Power
Booster/Vacuum Pump section.
(6) Rebuild booster vacuum reserve as follows: Re­
lease brake pedal. Increase engine speed to 1500
rpm, close throttle and immediately turn off ignition.
(7) Wait a minimum of 90 seconds and try brake
action again. Booster should provide two or more vac­
uum assisted pedal applications. I f vacuum assist is
not provided, perform booster and check valve vac­
uum tests. Also check vacuum output on diesel mod­
els.
POWER BOOSTER CHECK VALVE TEST
(1) Disconnect vacuum hose from check valve.
(2) Remove check valve and valve seal from
booster (Fig. 1). '
(3) Hand operated vacuum pump can be used for
test (Fig. 2).
(4) Apply 15-20 inches (50-67 kPa) vacuum at
large end of check valve (Fig. 1).
(5) Vacuum should hold steady. I f gauge on pump
indicates any vacuum loss, valve is faulty and must
be replaced.
BOOSTER
CHECK
Fig. 1 Vacuum Check Valve And Seal
J9005-80
Fig. 2 Typical Hand Operated Vacuum Pump
POWER BOOSTER VACUUM TEST
(1) Connect a vacuum gauge to the booster check
valve with a short length of hose and a T-fitting (Fig.
3).
(2) Start and run engine at idle speed for one
minute.
(3) Clamp hose shut between vacuum source and
check valve (Fig. 3).
(4) Stop engine and observe vacuum gauge.
(5) I f vacuum drops more than one inch vacuum
(33 millibars) within 15 seconds, either booster dia­
phragm or check valve are faulty.